Right now seeing a ship of another race you learn very little about it besides size, thermal and EM signature from your sensors, even if that ship is right in orbit of your planet.
I'd like some more things that can be learned about a ship short of getting the detailed blueprint, that can be learned from visual inspection of the hull.
Things that come to my mind that could be detected:
-Orbital Habitat Modules
-Miners, Fuel Harvesters, Maintenance Modules, Terraformers, etc.
-Cargo Holds, Troop Transports, Cryos
-Number of weapons
-Number of Turrets
-Number of Launchers
-Hangars
Detection could be handled by getting close to an active sensor, for example if a x HS target could be resolved, some of the ship features can be recognized.